Optical space-division switching fabric is expected to be a backbone of future optical switching systems because of its very large switching bandwidth. In general, there are two types of switching elements to implement optical space-division switching fabric: deflection-type switches and optical gates. The class of deflection-type switches, among which include mechanical switches, reflection switches, grating switches, directional coupler switches, X-switches, and mode-sorting switches, as discussed by R.C. Alferness in IEEE Transactions on Microwave Theory and Techniques, 1982, operate by directing an incoming optical signal to one of two output ports according to a control signal. Existing switch matrices based on these switch elements, including lattice, duobanyan, cross-bar, rectangular crossbar, full-active tree, and half-active tree matrices, are shown in FIGS. 1-6, respectively, as a set of 4.times.4 switch matrices. A general overview of these architectures is provided in the following publications: Voges and Neyer, IEEE/OSA Journal of Lightwave Technology, 1987; L. Thylen, IEEE/OSA Journal of Lightwave Technology, 1988; and R.C. Alferness, IEEE Journal of Selected Areas in Communications, 1988.
Contradistinctively, an optical gate switch as shown in FIG. 7 does not deflect the incoming optical signal. Rather, it divides the incoming optical signal among its two output gates wherein the switching is performed by the turn-on or turn-off of the gates. Switch matrices based on optical gates are basically a passive tree structure with an additional stage of optical gates. Himeno and Kobayashi in Electronic Letters, 1987, describe an optical-gate matrix switch using ON/OFF modulators such as Mach-Zehnder interferometers and cutoff modulators for optical gates. The optical gate can also be a switched laser diode amplifier, as suggested by M. Ikeda in Electronic Letters, 1981.
All of the above-mentioned switch structures, except the lattice structure, are strictly nonblocking matrices in which any unoccupied input/output ports can be connected without disturbing existing signal paths. Because several switches must be activated to set up a connection in a lattice matrix, some rearrangement of the switch activations may be required when new connections are requested. An obvious drawback is that some information is likely to be lost during this rearranging period. Accordingly, optical switch matrices based upon strictly nonblocking structures are usually preferred.
The nonblocking architectures still present difficulties despite their relative ease in connectivity. In the duobanyan and tree structures, more than one switch element must be activated to set up a connection, thereby requiring a more complicated control circuit which may slow down the switching set-up speed. In the crossbar matrices, the activation of only one switch element is sufficient to establish any designated point-to-point connection. However, the crossbar matrices are not suitable for the point-to-multipoint connections (or broadcasting) which are indispensable in fiber-optic video networks. The only deflection-device-based architecture that is suitable for point-to-multipoint connections is the half-active tree matrix.
With respect to the optical gate-type switch matrices, point-to-multipoint connections are inherent and switching is much faster and simpler than the tree matrices since each path is established by activating only one gate. The insertion loss of the gate-type switch matrices, however, is relatively high because they incur both splitting and combining optical losses. Recent progress in optical amplifiers have demonstrated that optical losses can be compensated by the amplifier gain. However, most optical switch matrices are still not practical because of the channel crosstalk and the need to individually adjust the drive voltages applied to the switch elements in order to obtain optimum operations.
As described hereinbefore, prior art configurations for optical switching matrices can be improved in terms of switching speed, traffic capacity, flexible connectivity, channel crosstalk, and control requirements. The evolution towards fiber-optic communication networks can be accelerated by an improvement in optical space switching fabric.